Recently, there are known high density magnetic recording media, which comprise a non-magnetic supporting substrate and a magnetic recording layer of a thin metal film formed thereon by means of vacuum deposition such as vacuum evaporation or sputtering or ion plating.
It is considered that metal films formed by a thin film-forming technique such as vacuum evaporation, sputtering, ion plating, etc. have a defect that the surface condition (unevenness) of the non-magnetic substrate appears on the surface of the magnetic layer as its own unevenness because of the thinness thereof, which causes degradation of electromagnetic conversion characteristics. From the viewpoint of electromagnetic conversion characteristics, it is desirable that the surface of non-magnetic substrate is as smooth as possible. However, there is a problem that smoothness of the substrate makes winding and unwinding of the medium difficult, makes poor the slipperiness between film and film and film and roll and is liable to cause defects such as scratch on the surface of the medium.
There have been made various attempts to make magnetic recording media which simultaneously satisfy the two above-described contradictory properties.
For instance, it was proposed to form, in the production line, a slip agent layer on at least one surface of a very smooth film and thus obtain a substrate, one of the surfaces of which is smooth and the other surface is slippery, and to form a magnetic layer on the smooth surface thereof. This method was not practical because defects such as flaw, etc. were caused during the casting and longitudinal stretching and thus the yield was poor.
Also it was proposed to form a laminated film of a smooth film and a slippery film by means of co-extrusion and further coat the slippery surface with a slip agent (Laid-Open Patent Publication No. 58-153640). Formation of a coating layer degrades the yield of the product. Therefore, a method which does not involve such coating procedure has been desired.
We conducted an extensive study in consideration of the above-described problems and found that an excellent magnetic recording medium can be obtained by employing a specific combination of layers and thus completed the present invention.